Continuous annealing process of steels exhibiting low yield point retarded of aging properties and good drawability

ABSTRACT

In a continuous annealing process for rimmed steels having &lt; OR = 0.06% carbon content, the coiling temperature of said steel while in a strip form, hot and rolling is more than 630*C to form massive carbides in said steel strip then after known coil reducing, is given a rapid cooling rate from a holding temperature during a continuous annealing stage of more than 50*C/sec, subesequent shelf treating temperature of said traveling strip is within the range of 300*C to 500*C for at least 10 seconds, and then said strip is coiled after cooling to room temperature, consequently low yield point of less than about 23kg/mm2, and retarded aging property and good drawability are exhibited.

United States Patent [191 Kubotera et al.

Nov. 4, 1975 OTHER PUBLICATIONS Garber, S.,A New Continuous Annealing Cycle for Blackplate, Special Report No. 79, The Iron and Steel Inst., 1963, pp. 81-86.

Primary ExaminerW. Stallard Attorney, Agent, or Firm-William Anthony Drucker [57] ABSTRACT In a continuous annealing process for rimmed steels having 0.06% carbon content, the coiling temperature of said steel while in a strip form, hot and rolling is more than 630C to form massive carbides in said steel strip then after known coil reducing, is given a rapid cooling rate from a holding temperature during a continuous annealing stage of more than 50C/sec, subesequent shelf treating temperature of said traveling strip is within the range of 300C to 500C for at least seconds, and then said strip is coiled after cooling to room temperature, consequently low yield point of less than about 23kg/mm and retarded aging property and good drawability are exhibited.

5 Claims, 3 Drawing Figures CONTINUOUS ANNEALING PROCESS OF STEELS EXHIBITING LOW YIELD POINT RETARDED OF AGING PROPERTIES AND GOOD DRAWABILITY [76] Inventors: Haruo Kubotera, 371 Kami-Kurata, Totsuka-ku, Yokohama, Kanagawa; Kazuhide Nakaoka, No. 1030 Yagami, Kawasaki, Kanagawa; Kaoru Watanabe, No. 1497-29, Iikuraya, Shinohara, Kohoku Yokohama, Kanagawa; Akihiko Nishimoto, No. 2412 Kami-Wada, Yamato, Kanagawa; Nobuo Tanaka, 1-3, 3-Chome, Isegaoka, Fukuyama, Hiroshima, all of Japan [22] Filed: Oct. 26, 1973 [21] Appll No.: 410,160

Related US. Application Data [63] Continuation-inpart of Ser. No. 115,474, Feb. 16,

1971, abandoned.

[52] US. Cl 148/12 C [51] Int. Cl. C21D 9/48 [58] Field of Search 148/12 C, 12.3

[56] References Cited UNITED STATES PATENTS 2,381,435 8/1945 Burns et a1 148/12 3,412,781 11/1968 Richards 164/76 TIME (second) CONTINUOUS ANNEALING PROCESS OF STEELS EXHIBITING LOW YIELD POINT RETARDED OF AGING PROPERTIES AND GOOD DRAWABILITY CROSS REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of application, Ser. No. 1 15,474, filed Feb. 16, 1971, now abandoned.

This invention relates to a complete continuous annealing process for rimmed steels exhibiting good drawability, and more particularly steel exhibiting low yield point and retarded aging properties.

For the most part, the materials made by means of the known continuous annealing process has been scarely employed as drawing services, while said material exhibits a long aging properties and high hardness. It is well known that the reason mainly lies in a far more rapid cooling process than that of the batch type annealing process. That is, said rapid cooling process brings about an increase of solution carbon in steel.

Therefore, many studies for improving the quanity of steels made by said continuous annealing process have been made. The US. Steel Process in USA. and the Dortmund Process in West Germany may be cited as typical cases of these studies. The former is characterized by shelf treatment on travelling steel strip and the later is characterized by overaging treatment on coiled steel. These processes are reported as techniques which can improve the workability of hot-dip galvanizing steel. It may be said, however, that said improvements merely lie a decrease of the hardness of the steel. It is, especially, well known that productivity of the latter is very low because of using a batch type annealing process. In any case, it will be sure to report nonimprovement of drawability, especially, aging property of the steel.

The present invention has been developed to improve said drawability and said aging property by means of the continuous annealing process, which is possible to create a high productivity.

The features of the present invention lie in, at first, that the carbon in a rimmed steel is perpared to be 0.06% or less, secondly, a hot rolled strip of said steel is coiled at more than 630C to form massive carbides, thirdly, said steel strip subjected to continuously annealing after cold reducing is rapidly cooled from a heating-up temperature of recrystallization temperature to A point with a rapid cooling rate of more than 50C/sec, fourthly, subsequent shelf treatment is carried out at a temperature of 300C to 500C for at least seconds, finally, said self-treated strip is coiled to room temperature and is coiled.

In accordance with the above mentioned process, a rimmed steel having a low yield point of less than 23kg/mm retarded aging property, and good drawability is possible to be obtained with ease.

Thus, an object of this invention is to provide animproved continuous annealing process of a rimmed steel exhibiting good drawibility.

Another object of this invention is to provide an improved continuous annealing process of a rimmed steel exhibiting retarded aging properties.

A further object of this invention is to provide an improved continuous annealing process of a rimmed steel exhibiting press-formbility with specially low yield point of less than 23kg/mm Other objects and advantages will become apparent from the following detailed description and with the accompanying drawings, in which:

FIG. 1 shows an example of the present continuous annealing cycle in comparison with the usual cycle.

FIG. 2 is a modification of said cycle shown in FIG. 1 wherein the steel is rapidly cooled to room temperature.

FIG. 3 is a diagram illustrating the interrelation between yield point or aging index and shelf treating temperature.

In the present process, it is desirable that the amount of carbon in the steel is limited to S 0.06%. It is, generally, a fact that the yield point has a strong depend ability on the carbon content in the steel, especially in the steel made by said continuous annealing process compared with batch type annealing process. For this reason, said carbon content is restricted to the above range. According to the Applicants experiments, it was discovered that said yield point of steel having the above carbon content can be effectively lowered in. comparison with other steel having the same level of said carbon content, which is made by the ordinary process, when the steel having a carbon content of 5 0.06% is coiled at an elevated temperature of more than 630C. Such a coiling temperature gives an impetus to grain growth and enlargement of carbide, i.e. formation of massive carbide, at the present continuous annealing stage as mentioned later. It has been confirmed that said yield point after the present continuous annealing is remarkably lowered to less than 23kg/mm The present process is employed with a rimmed steel. There are no limit of other component excepting carbon. All of the known processes for obtaining reduced steel strip i.e. steel making process, ordinary casting process, continuous casting process, slabbing process, common hot rolling process, and cold reducing process, may be employed.

The present complete continuous annealing cycle is as shown in FIG. 1 and FIG. 2. In these figures, numeral I shows said cycle and numeral II, the ordinary cycle. The present heating-up rate can be carried out as said ordinary cycle II stands, while the quanity and aging property of said steel are little influenced by how is heated said strip. The holding temperature is within the range of more than recrystallization temperature to A ,Point, preferably, 710C to 750C. The holding time is same as that of the ordinary cycle, which is sufficient to uniformly heat said strip. And then it needs the rapid cooling rate of more than 50C per second in order that said strip reaches the shelf treating temperature as mentioned later or room temperature. The cooling rate of less than 50C per second is undesirable, because said aging index of the steel becomes higher than that of batch type annealing steel.

FIG. 3 shows interrelation between the shelf treating temperature and said aging index or yield point. Referring now to FIG. 3, it will be well inderstood that said shelf treating temperature should be within the range of about 300C to 500C. Specially temperatures of 350C to 450C bring about the lowest aging index and yield point. Such features are evident by the curve [I] of said yield point and the curve IV of said aging index. When said strip is shelf-treated for at least 10 seconds, the above-mentioned properties can be obtained with ease. Such serial treatments are carried out on said travelling steel strip. Said strip is cooled to room temperature 3 under ordinary manner and then coiled.

Actual embodiments of this invention steels are as follows:

4 range, i.e. c 0.06% as mentioned above. It is needless to say that the mechanical properties of steel that have said carbon contentfi. 'steel 5, and steel 6, is out of the present range and-"is far inferior to that of the pres- Referring to the above Tables, it will be understood that the mechanical properties of the present steels, i.e.

Steel making conditions 5 ent steel, even thdugh th'econtinuous annealing proc cgnveror cess of steel 5- A aridisteel 6-A. ca ompos' as at The reasons that said mechanical properties of the. present steel is be tter than that of other steels lie in the limitin of the carbon content, the coiling temperature Table I g I l C 1 10 at the hot rolling stage, the rapid cooling rate and the tee P s N O shelf treatment at a continuous annealing stage. Spe- 5 31 0.29 0.012 0.016 0.0012 0.056 cially with a rapid cooling rate said saturated solution 3 2; 8:33 8:8}; 8:55;; 8:88}; 8:8 carbon is remarkably increased and in the subsequent 4 0.060 0.33 0.011 0022 0001 ,03 shelf treating stage precipitation of such a carbon is g 8 033 (123 (1016 rapidly accelerated. Thus, a low yield point of less than N .0 0.34 0.015 0.018 0.0015 0.039 23 K 1 2 d d d fl th 5 g/mm an retar e aging property 0 ess an K mm that is lower than that of the usual batcht e I 6 I I Hot rolling conditions after Slabbing, annealing steel are possible to be obtained w1th ease, as finishing temperatures: 870C w coiling temperatures: 700C at c i finishing thickness 2 8mm I 1. In a continuous annealing process for producing Cbldreducing conditions after pickling steel having a yield strength of less than 23 Kglmm a reducing rate 7 1 4% retarded aging property and good drawability, compristhickne'ssz OSmming passing a low carbon'steel of S 0.06 carbon seri- Annealin-g conditions, 25 ally through a heating zone, rapid coolingzone, shelf cycle A: this invention cycle with continuous annealireatmg l and coolmg Zone the fnpmvemem which ing process t a 7 includes coiling a hot rolled steel strip at a temperature Soaking temperature, 750C of more than 630C. and then quenching the strip from cooling rater 1200C c the recrystallization"temperature to a temperature be- 'Shlf treating, 400CX 30 tween room temperature and 500C at a rate of more cycle Bi usual continuous annealing process soaking h C/Sec m .Sald iapld coolmg f temperature, 750C 2. A complete cont nuous annealing process as set cycle usual'bat'ch type annealing process forth in claim 1 wherein said carbon content is within soaking temperature: 700C 35 the range of '0 t I Y Soaking ti m e: 6hr I r 3. complete continuous annealing process as set Mechanical properties after above memioned forth in claim 1 wherein said coiling temperature at hot treating, as under I rolling stage is about 700C.

t T5616 11 Steel Cycle Yield Tensile Total Aging Lankford Point Strength Elongaindex Value (Kg/mm") (Kg/mm tion' (Kg/mm) (1') A v 19.8 31.3 46.1 3.5 1.33 1 B 24.3 33.9 41.3 7.1 1.31 C 23.4 31.7 46.8 4.4 1.36 A 20.4 31.9 45.3 3.8 1.26 2 B 25.2 34.0 40.6 7.1 1.26 p C 22.5 32.0 45.1 4.9 1.29 A 21.0. 31.8 45.2 3.9 1.31 3 I B 26.8 35.6 40.1 6.8 1.25 C 21.9 32.1 44.9 5.1 1.38 A 20.7 32.0 45.0 4.1 1.29 4' 13 27.3 36.0 40.4 6.8 1.28 C 22.1 32.4 44.3 5.5 1.34 A 22.9 33.9 44.1 4.3 1.23 A 5 B 28.7 37.5 40.0 a 6.9 1.24 C 23.8 33.1 44.0 5.2 1.22 A 24.5 34.3 42.5 4.1 1.21 6 B 29.8 38.1 38.0 7.0 1.25 C 24.1 33.5 43.8 5.1 1.20

4. A complete continuous annealing process set forth in claim 1 wherein said holding temperature of the steel l-A, 2-A, 3-A and 4-A steels, are superior to that of the strip is-within the range of 710C to 750C.

ordinarycontinuous annealing steels, i.e. l-B, 2 -B, 3-B and 4-B steels, and is similar to that of the usual batch annealing steels, i.e. l-C, 2-C, 3-C and 4-C steels, while thecarbon content of these steels is withinv the present I 5. Acomplet'e continuous annealing process set forth in claim 1 wherein said shelf treating of the steel strip is carried-out within the range of 350C to 450C. 

1. IN A CONTINUOUS ANNEALING PROCESS FOR PRODUCING STEEL HAVING A YEILD STRENGTH OF LESS THAN 23 KG/MM2, A RETARDED AGING PROPERTY AND GOOD DRAWABILITY, COMPRISING PASSING A LOW CARBON AND STEEL OF $ 0.06% CARBON SERIALLY THROUGH A HEATING ZONE, RAPID COOLING ZONE, SHELF TREATING ZONE AND COOLING ZONE, THE IMPROVEMENT WHICH INCLUDES COILING A HOT ROLLED STEEL STRIP AT A TEMPERATURE OF MORE THAN 630*C.AND THEN QUENCHING THE STRIP FRON THE RECRYSTALLIZATION TEMPERATURE TO A TEMPERATURE BETWEEN ROOM TEMPERATUE AND 500*C AT A RATE OF MORE THAN 50*C/SEC IN SAID RAPID COOLING ZONE.
 2. A complete continuous annealing process as set forth in claim 1 wherein said carbon content is within the range of 0.03 to 0.06%.
 3. A complete continuous annealing process as set forth in claim 1 wherein said coiling temperature at hot rolling stage is about 700*C.
 4. A complete continuous annealing process set forth in claim 1 wherein said holding temperature of the steel strip is within the range of 710*C to 750*C.
 5. A complete continuous annealing process set forth in claim 1 wherein said shelf treating of the steel strip is carried out within the range of 350*C to 450*C. 